Some types of partial middle ear implantable (P-MEI), total middle ear implantable (T-MEI), cochlear implant, or other hearing assistance systems utilize components disposed within the middle ear or inner ear regions. Such components may include an input transducer for receiving sound vibrations or an output stimulator for providing mechanical or electrical output stimuli corresponding to the received sound vibrations.
An example of such a device is disclosed in U.S. Pat. No. 4,729,366, issued to D. W. Schaefer on Mar. 8, 1988. In the '366 patent, a mechanical-to-electrical piezoelectric input transducer is associated with the malleus bone in the patient's middle ear. The malleus vibrates in response to sounds received at the patient's tympanic membrane (eardrum). The piezoelectric input transducer transduces mechanical energy of malleus vibrations into an electrical signal, which is amplified and further processed by an electronics unit. A resulting electrical signal is provided to an electrical-to-mechanical piezoelectric output transducer that generates a mechanical vibration coupled to an element of the ossicular chain or to the oval window or round window. In the '366 patent, the ossicular chain is interrupted by removal of the incus. Removal of the incus prevents the mechanical vibrations delivered by the piezoelectric output transducer from mechanically feeding back to the piezoelectric input transducer.
Hearing assistance systems that sense sounds through corresponding ossicular vibrations face numerous difficulties. For example, the direction of motion of malleus vibrations varies unpredictably between individuals, making such vibrations difficult to transduce into a corresponding electrical signal, since sensors that transduce such vibrations typically perform optimally if the motion of the vibrations is in a particular direction. For example, a cantilevered piezoelectric bimorph input transducer provides a maximum output electrical signal amplitude when the direction of motion of the vibrations is perpendicular its longitudinal direction. A further complication is that the direction of motion of the malleus vibrations may be frequency dependent. As a result, an input transducer that is optimally positioned to sense malleus vibrations at a particular frequency in the audio frequency range may be inadequately positioned to sense malleus vibrations at a different frequency in the audio frequency range. There is a need in the art to provide sensing of sound vibrations in the inner ear, in which any variability in direction of motion of ossicular or other auditory elements is accommodated.